1. Field of the Invention
The invention relates generally to the field of photography, and in particular to a film cassette containing a film spool about which a filmstrip is coiled. More specifically, the invention relates to a film cassette having a spool lock for preventing rotation of the film spool and a light-shielding door that can be opened to permit film movement out of and back into the cassette interior.
2. Description of the Prior Art
U.S. Pat. No. 5,115,268 issued May 19, 1992 discloses a film cassette comprising a light-trapping plush for preventing ambient light from entering the cassette interior through a film egress/ingress passageway, a film spool supported for rotation in film unwinding and film winding directions, and a locking pawl supported for movement into engagement with the spool to secure the spool and out of engagement with the spool to release the spool. The spool has a coaxial ratchet wheel with assymmetrically shaped teeth. The locking pawl appears to be a rigid beam member which is pivotally mounted at one end and has an assymmetrically shaped detent portion at an opposite end. A curved spring arm portion of the locking pawl extends from the end of the rigid beam member that has the detent portion, to bias the detent portion to between any two adjacent ones off the teeth in order to secure the spool. To remove the detent portion from between two adjacent ones of the teeth, an engagement release portion of the locking pawl is turned to pivot the rigid beam member against the contrary urging of the spring arm portion. The engagement release portion then is held in the turned-to position to maintain the detent portion separated from the ratchet wheel. When the engagement release portion is released, the spring arm portion is free to pivot the rigid beam member to return the detent portion to between any two adjacent ones of the teeth in order to re-secure the spool.
Problems to be Solved by the Invention
Since in U.S. Pat. No. 5,115,268 the beam member of the locking means is apparently rigid, and the teeth of the ratchet wheel and the detent portion of the beam member are assymmetrically shaped, an attempt to forcibly rotate the spool in a film unwinding direction will most likely fracture the beam member.
Also, in the '268 patent the film cassette is of the type that is capable of advancing a non-protruding film leader outwardly from the cassette interior when the film spool is rotated in the unwinding direction. However, it is possible that the leading end of the film leader will stub against the light-trapping plush, in which instance the film leader cannot be advanced out of the cassette interior. Moreover, should the film leader be moved out of and back into the cassette interior several times, this may cause the plush to lose its effectiveness as a light-trapping means.